This application claims the priority of German Patent Application, Serial No. 199 47 439.7, filed Sep. 24, 1999, the subject matter of which is incorporated herein by reference.
The present invention relates to an apparatus for withdrawing a screw from the barrel of a screw extruder.
Typically, a screw extruder has a barrel which defines a bore for receiving a screw. During operation, the screw rotates in the barrel bore to convey a medium, and has a head portion, called screw cap, which is formed with an outwardly pointing undercut. In general, the screw of a single screw extruder or the screws of a multi-screw extruder have a geometric shape that is best suited to the particular properties of a material or material group that is processed in the extruder. As a consequence, each time when the extruder is intended for use with a different material of markedly different properties, the screw must be replaced for a different type to ensure optimum operating conditions. Normally, the screw exchange is carried out in such a manner that the screw is extracted in conveying direction, i.e. the screw is withdrawn at the head portion of the extruder from the barrel bore. Oftentimes, the screw is composed of a plurality of components which are arranged axially behind one another and formed with a longitudinal bore for passage of a shaft. Threadably mounted onto the shaft is the screw cap which forms a closing element in conveying direction and braces the individual components of the screw together. The screw cap thus forms the head portion of the screw. As substantial forces must be applied, especially when extruders of larger size are involved, in order to extract the screw from the barrel bore, particular pull mechanisms, such as chain tackles or hydraulic cylinders, have frequently been used for attachment to the head portion. Normally, the head portion is provided with a circumferential groove in the form of an outwardly facing undercut for positive engagement of gripper claws of an extractor device, and a post-processing device, such as a screen pack changer, a die plate of a granulating machine, slot die or other nozzle for producing extrudate, is operatively connected to the head portion of the extruder. As a consequence of the comparably robust configuration of the gripper claws, conventional extraction apparatus require a dismantling of the post-processing device before a screw replacement can take place so as to create the needed space for attachment of the extraction apparatus. Dismantling and alterations are fairly cumbersome and increase the manufacturing costs, in particular when smaller batches of different material are processed as a screw replacement is needed within shorter intervals compared to larger batches that run at respectively longer operating periods, without requiring a screw replacement.
It is thus an object of the present invention to provide an improved screw extraction apparatus, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved screw extraction apparatus which is simple in structure and yet is capable to significantly reduce the complexity of replacing a screw of a screw extruder.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by providing at least one gripper having a free end for attachment of a pull mechanism and a grabbing end for interaction with an undercut of the screw cap, with the at least one gripper being formed at grabbing end with a plurality of radially resilient clamping segments which can be elastically bent inwards and/or outwards, and which form parts of a hollow cylinder in longitudinal direction thereof, whereby the hollow cylinder has an inner diameter, which substantially corresponds to an outer diameter of the screw cap, and an outer diameter which is smaller than a diameter of a bore of the barrel, wherein each of the clamping segments terminates in a beaded end which extends radially into the hollow cylinder for formation of a gripping claw, with the beaded end having a wall thickness which is smaller than a difference between a radius of the barrel bore and an outer radius of the screw cap, and, in longitudinal direction of the gripper, a length which is smaller than a width of the undercut of the screw cap in longitudinal direction of the screw.
The novel and inventive approach of the present invention is based on the recognition that the complexity of a screw replacement can be substantially reduced when a dismantling of post-processing devices, positioned downstream of the extruder head, can be eliminated. To date, conventional extraction apparatuses did not allow this approach because, without dismantling of the post-processing device, there was simply not enough space available to attach the extraction apparatus. This problem has now been overcome through the provision of an extraction apparatus according to the present invention, which exhibits a very slim and extremely space-saving configuration of its gripper tools.
Through the provision of a gripper with radially resilient clamping segments in accordance with the present invention, the gripper end can be inserted into the barrel bore and pushed over the screw cap, whereas the beaded ends of the clamping segments are so configured in size and shape as to form gripping claws which are prevented from wedging in the ring gap between the screw cap and the barrel bore. When the gripper end is pushed sufficiently far over the screw cap, the beaded ends can positively engage or snap in the undercut. Depending on the magnitude of the pulling forces applied in axial direction by means of a pull mechanism, such as a hydraulic cylinder, attached to the extraction apparatus, and the magnitude of the spring force exerted by the clamping segments, the positive engagement of the beaded ends in the undercut may be enough by itself in some cases to allow a withdrawal of the screw from the barrel bore. However, normally, it may be more advantageous to provide the gripper with a securing sleeve by which an inadvertent outward deflection of the clamping segments and thus a release of the positive engagement are prevented when axial forces are applied to extract the screw. The securing sleeve is suitably shiftable over the gripper up to the area of the clamping segments to thereby prevent the outward deflection of the clamping segments. Suitably, the securing sleeve has an inner diameter, which corresponds to the outer diameter of the hollow cylinder so as to be easily slideable over the hollow cylinder, and an outer diameter, which is smaller than the inner diameter of the barrel bore so that the securing sleeve can easily be pushed into the barrel with accommodated screw.
Suitably, the gripper has, as an essential component, a tubular portion, whereby the clamping segments are formed by a plurality of slits extending in axial direction from the grabbing end inwardly of the tubular portion. The beaded ends may be formed by selecting for the grabbing end a tubular portion with excess wall thickness, and subsequently turning the wall thickness from the inside to size, except in the area that should form the beaded ends. Of course, there is no need to reduce the wall thickness across the entire length of the tubular portion. It is sufficient to turn the wall thickness of the tubular portion to size along a section that is long enough to allow the inner surface of the tubular portion to contact the screw cap. Suitably, the turned section extends slightly beyond the length of the clamping segments so that the clamping segments have a smaller wall thickness than the rest of the tubular portion.
To facilitate a detachment of the screw extraction apparatus, after withdrawal of the screw and release of the clamping segments through retraction of the securing sleeve, i.e. to allow a fairly easy outward deflection of the clamping segments when applying axial pulling forces, it is proposed to provide the gripper on the inside with a circumferential bevel at the clamping segments in a transitional area between the wall thickness of the hollow cylinder and the beaded end. In this way, the applied pulling forces automatically generate radially outwardly acting deflection forces to outwardly deflect and open the clamping segments as a consequence of the wedge effect of the bevel.
Of course, it may be suitable to so configure the clamping segments in a rest position or disengaged state as to slightly bend up radially outwards so that the inner diameter of the gripper approximately corresponds in the area of the beaded ends to the outer diameter of the screw cap or even slightly exceeds the outer diameter of the screw cap in which case the extraction apparatus can be detached after disengagement of the clamping segments directly from the screw cap, without a need to apply a particular force. In order for the clamping segments to seize in this case the screw cap, once the extraction apparatus is attached, and thus to realize the positive engagement of the beaded ends in the undercut of the screw cap, the required radially inward deflection of the clamping segments is effected by the securing sleeve when pushed over the clamping segments.
The screw extraction apparatus according to the present invention is particularly useful for withdrawing a screw of a twin screw extruder. Accordingly, the extraction apparatus has two grippers having longitudinal axes extending in side-by-side parallel disposition at a distance which corresponds to a distance of the two screws. Both grippers are braced together by a connecting member for retaining their relative alignment. Although it may certainly be possible to use two grippers which are configured completely separate from one another, the use of a twin aggregate with interconnected grippers is preferred. The connecting member for bracing the two grippers may be comprised of two tie rods and a first yoke, with the tie rods and the yoke being joined together by bolting or welding. As the use of securing sleeves is also preferred in this case, these securing sleeves are each formed by a tubular portion and joined together by a second yoke so as to establish a common movement. Of course, it is certainly possible to omit such an interconnection of the securing sleeves altogether and to move the securing sleeves in succession to their respective locking position.
Suitably, the second yoke may have throughholes for the tie rods of the first yoke to thereby establish a displacement of the securing sleeves in a precise parallel relationship. The first yoke for interconnecting the two grippers may be connected to an eyelet for easier attachment of the pull mechanism. The eyelet may be secured to a threaded rod received in a threaded bore of the first yoke and extending to the second yoke, with the threaded rod being connected to the second yoke in such a manner that the threaded rod can turn in the second yoke but is immobile in an axial direction relative to the second yoke. In this way, a turning of the eyelet is translated into a threading of the threaded rod through the first yoke to thereby effect a respective longitudinal displacement of the securing sleeves relative to the gripper. Thus, a manual operation of the extraction apparatus can be carried out effortlessly to implement a locking and unlocking of the grippers.
Suitably, the gripper is made of a metal, preferably steel so as to be able to withstand comparably high loads with the required small wall thickness of the extraction apparatus. Of course, some or all components can also be made from other suitable materials, such as fiber composites (e.g. carbon fibers).
Through the provision of a screw extraction apparatus according to the present invention, a screw replacement can be carried out, without requiring a time-consuming and complicated dismantling of post-processing devices that are attached to the extruder head. For example, in case of a screen pack changer as post-processing device, there exists a situation between the operational states of both screen elements in which the screw cap of the extruder screw is accessible from outside. In this position, the extraction apparatus according to the present invention can reach the screw cap through the screen pack changer and, after engagement of the clamping segments in the undercut of the screw cap and, optionally, locking of the clamping segments by the securing sleeve, the screw can be withdrawn from the barrel bore of the extruder through the screen pack changer. The same is true in the event the extruder head is connected to an extrusion die. When, for example, the nozzle head is guided by a rail system for displacement in a longitudinal direction, transversely to the extrusion direction, upon an intermediate plate which is securely fixed to the extruder, as described, for example, in German Pat. No. DE 195 04 119 A1, the nozzle head can be quickly released and easily pushed aside so that the screw cap of the extruder screw becomes again accessible from outside. Due to the fact that the intermediate plate remains securely fixed to the extruder, the available free working space is still be too small for attachment of conventional extraction apparatuses so that a dismantling of the intermediate plate would be needed. This, however, is not necessary when using an extraction apparatus according to the present invention, as the novel and inventive extraction apparatus can reach through the extrusion space of the intermediate plate to the screw head so that the extruder screw can be extracted to the outside through the intermediate plate.
A screw extraction apparatus according to the present invention can be fabricated in a comparably simple manner and is easy to operate. As a consequence of the small wall thickness of the gripper and the securing sleeves, the outer dimensions only slightly exceed the dimensions of the extruder screw. Yet, the extraction apparatus is very robust and capable to absorb great forces, without risking an inadvertent opening of the locked connection between the screw caps and the grippers.